Re: [PATCH v5 00/15] x86: Add support for Clang CFI

Peter Zijlstra
2021-10-27T14:03:47Z

On Wed, Oct 27, 2021 at 03:30:11PM +0200, Ard Biesheuvel wrote:

> As far as I can tell from playing around with Clang, the stubs can
> actually be executed directly,

I had just finished reading the clang docs which suggest as much and was
about to try what the compiler actually generates.

> they just jumps to the actual function.
> The compiler simply generates a jump table for each prototype that
> appears in the code as the target of an indirect jump, and checks
> whether the target appears in the list.
>
> E.g., the code below
>
> void foo(void) {}
> void bar(int) {}
> void baz(int) {}
> void (* volatile fn1)(void) = foo;
> void (* volatile fn2)(int) = bar;
>
> int main(int argc, char *argv[])
> {
> fn1();
> fn2 = baz;
> fn2(-1);
> }
>
> produces
>
> 0000000000400594 <foo.cfi>:
> 400594: d65f03c0 ret
>
> 0000000000400598 <bar.cfi>:
> 400598: d65f03c0 ret
>
> 000000000040059c <baz.cfi>:
> 40059c: d65f03c0 ret

Right, so these are the actual functions ^.

> 00000000004005a0 :
> 4005a0: a9bf7bfd stp x29, x30, [sp, #-16]!
>
> // First indirect call
> 4005a4: b0000088 adrp x8, 411000 <__libc_start_main@GLIBC_2.17>
> 4005a8: f9401508 ldr x8, [x8, #40]
> 4005ac: 90000009 adrp x9, 400000 <__abi_tag-0x278>
> 4005b0: 91182129 add x9, x9, #0x608
> 4005b4: 910003fd mov x29, sp
> 4005b8: eb09011f cmp x8, x9
> 4005bc: 54000241 b.ne 400604 <main+0x64> // b.any
> 4005c0: d63f0100 blr x8

That's impenetrable to me, sorry.

> // Assignment of fn2
> 4005c4: 90000009 adrp x9, 400000 <__abi_tag-0x278>
> 4005c8: b0000088 adrp x8, 411000 <__libc_start_main@GLIBC_2.17>
> 4005cc: 91184129 add x9, x9, #0x610
> 4005d0: f9001909 str x9, [x8, #48]

I'm struggling here, x9 points to the branch at 400610, but then what?

x8 is in .data somewhere?

> // Second indirect call
> 4005d4: f9401908 ldr x8, [x8, #48]
> 4005d8: 90000009 adrp x9, 400000 <__abi_tag-0x278>
> 4005dc: 91183129 add x9, x9, #0x60c
> 4005e0: cb090109 sub x9, x8, x9
> 4005e4: 93c90929 ror x9, x9, #2
> 4005e8: f100053f cmp x9, #0x1
> 4005ec: 540000c8 b.hi 400604 <main+0x64> // b.pmore
> 4005f0: 12800000 mov w0, #0xffffffff // #-1
> 4005f4: d63f0100 blr x8
>
>
> 4005f8: 2a1f03e0 mov w0, wzr
> 4005fc: a8c17bfd ldp x29, x30, [sp], #16
> 400600: d65f03c0 ret
> 400604: d4200020 brk #0x1

> 0000000000400608 <__typeid__ZTSFvvE_global_addr>:
> 400608: 17ffffe3 b 400594 <foo.cfi>
>
> 000000000040060c <__typeid__ZTSFviE_global_addr>:
> 40060c: 17ffffe3 b 400598 <bar.cfi>
> 400610: 17ffffe3 b 40059c <baz.cfi>

And these are the stubs per type.

> So it looks like taking the address is fine, although not optimal due
> to the additional jump.

Right.

> We could fudge around that by checking the
> opcode at the target of the call, or token paste ".cfi" after the
> symbol name in the static_call_update() macro, but it doesn't like
> like anything is terminally broken tbh.

Agreed, since the jump table entries are actually executable it 'works'.

I really don't like that extra jump though, so I think I really do want
that nocfi_ptr() thing. And going by:

https://clang.llvm.org/docs/ControlFlowIntegrityDesign.html#forward-edge-cfi-for-indirect-function-calls

and the above, that might be possible (on x86) with something like:

/*
* Turns a Clang CFI jump-table entry into an actual function pointer.
* These jump-table entries are simply jmp.d32 instruction with their
* relative offset pointing to the actual function, therefore decode the
* instruction to find the real function.
*/
static __always_inline void *nocfi_ptr(void *func)
{
union text_poke_insn insn = *(union text_poke_insn *)func;

return func + sizeof(insn) + insn.disp;
}

But really, that wants to be a compiler intrinsic.